Fuel assemblies in nuclear reactors are subject to hydraulic forces that may exceed the weight of the fuel assemblies and therefore cause the fuel assemblies to "float" in the reactor if they are not properly secured. If a fuel assembly were to float upward just enough to cause it to be disengaged from the seating surface of the lower core plate on which it sits, it would in essence be laterally unrestrained, and this condition could subject the fuel assembly to severe fretting. Because of this possibility, fuel-assembly designs have included elements whose purpose is to prevent floating.
One method of preventing floating is to mount springs on the tops of the fuel assemblies. The spring are compressed between an upper plate and the remainder of the fuel assembly, thereby providing sufficient holddown force to prevent the fuel assembly from being disengaged from seating surfaces on the lower core plate. While this method has been effective in the prevention of floating, designs employing it have characteristics that make it desirable to have the alternative of the present specification available.
Among such characteristics is that the springs apply a compressive force to the structure of the fuel assembly. This compressive force makes it necessary to build assemblies that are strong enough to withstand the expected compressive forces, and the increased strength requirements dictate that more material be used in the structural (non-fuel) parts of the assembly. This increased amount of material results in increased drag, thereby increasing size and force requirements on the springs. Accordingly, the returns to be derived from increasing the size of the springs diminish as the holddown force required increases. Also, higher compressive loads on the fuel-assembly structure increase the tendency of the fuel assembly to bow due to thermal-/and irradiation-induced creep of the structural material. This bowing, which can cause refueling difficulties and higher fuel temperatures, is particularly likely to occur when the structural parts of the assembly are made of Zircaloy, which exhibits a fairly strong tendency for growth and creep.
The spring problem is further complicated by the fact that length variations in the various components of the nuclear reactor system occur during reactor operation, and this makes it necessary that the springs be designed to apply the rated force even when such length variations allow the springs to extend to a relatively expanded condition. But if the springs apply the rated force when they are in a relatively expanded condition, then they apply an even greater force when length variations cause them to contract, and this again requires that the fuel assembly include a greater quantity of structural material. Accordingly, if length variations are to be expected, then either the fuel assemblies must be designed to bear a greater compressive load, or the springs must be made long enough so that length variations do not contribute greatly to the Hooke's-Law force. Neither of these requirements is welcomed by the fuel-assembly manufacturer.
Since current fuel-assembly designs are complicated by length variations and are quite adversely affected by increased flow rates, it has been suggested that improved results could be obtained by holding the fuel assemblies down from their lower, or upstream, end, using the lower core plate as a source of holddown force; this scheme would eliminate the compressive force on the fuel assembly. While this idea may ultimately prove workable, it would not be undesirable to have an alternate design that avoids two of the design requirements dictated by this scheme. These are that a reliable latching mechanism be provided and that the core barrel be designed to have enough strength to withstand the additional load applied to it by the lower core plate.